create flake8 config, update travis

.flake8

@@ -0,0 +1,8 @@
+[flake8]
+# Ignore list taken from https://github.com/psf/black/blob/master/.flake8
+# E203	Whitespace before ':'
+# E266	Too many leading '#' for block comment
+# E501	Line too long (82 > 79 characters)
+# W503	Line break occurred before a binary operator
+ignore = E203, E266, E501, W503
+exclude = docs, build

.travis.yml

@@ -1,6 +1,5 @@
 dist: xenial
 language: python
-sudo: false
 os:
   - linux
 cache:
@@ -13,15 +12,12 @@ python:
 install:
   - pip install codecov
   - pip install .[dev]
-  - if [[ $TRAVIS_PYTHON_VERSION == 3.6 ]]; then pip install -U black; fi
+  - if [[ $TRAVIS_PYTHON_VERSION != 3.5 ]]; then pip install -U black; fi
 script:
   - export PYTHONPATH=$PYTHONPATH:$(pwd)
-  # * E501 - line length limit
-  # * E123 - indentation on data structures
-  # * W504 - line break after binary operator
-  - flake8 . --ignore=F403,E501,E123,E128,W504,W503 --exclude=docs,build
+  - flake8 .
   - py.test --cov=mesa tests/ --cov-report=xml
-  - if [ $PYTHON == "3.6" ];
+  - if [ $PYTHON != "3.5" ];
     then black --check --exclude=mesa/cookiecutter-mesa/* .;
     fi
   # - ./tests/test_end_to_end_viz.sh  # needs to be investigated on why this took forever

mesa/agent.py

@@ -13,7 +13,7 @@
 class Agent:
     """ Base class for a model agent. """
 
-  def __init__(self, unique_id: int, model: Model) -> None:
+    def __init__(self, unique_id: int, model: Model) -> None:
         """ Create a new agent. """
         self.unique_id = unique_id
         self.model = model